Journal of Petrology, Volume 40, Issue 9: September 1999.

Post-Collisional Potassic and Ultrapotassic Magmatism in SW Tibet: Geochemical and Sr-Nd-Pb-O Isotopic Constraints for Mantle Source Characteristics and Petrogenesis

C. MILLER¹, R. SCHUSTER², U. KLÖTZLI², W. FRANK² AND F. PURTSCHELLER¹

¹INSTITUT FÜR MINERALOGIE UND PETROGRAPHIE, INNRAIN 52, A-6020 INNSBRUCK, AUSTRIA
²INSTITUT FÜR GEOLOGIE, ALTHANSTRASSE, A-1090 WIEN, AUSTRIA

Major and trace element, Sr-Nd-Pb-O isotope and mineral chemical data are presented for post-collisional ultrapotassic, silicic potassic and high-K calc-alkaline volcanic rocks from SW Tibet, with ⁴⁰Ar/³⁹Ar ages in the range 17-25 Ma. The ultrapotassic lavas contain mantle xenocrysts (olivine ± rutile/armalcolite). Their initial ⁸⁷Sr/⁸⁶Sr (0·7172-0·7220) and ¹⁴³Nd/¹⁴⁴Nd (0·51190-0·51200) ratios suggest that they originated from lithospheric sources enriched in Rb with low Sm/Nd ratios. Initial Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 18·41-18·51; ²⁰⁷Pb/²⁰⁴Pb = 15·68-15·72; ²⁰⁸Pb/²⁰⁴Pb = 39·42-39·60) and geochemical features such as high Th/Ta, low Sr/Nd, low Ce/Pb and negative Eu anomalies are consistent with a recycled crustal component. Nd depleted mantle model ages range from 1·3 to 1·9 Ga, whereas Pb model ages record an Archaean event, suggesting that the source had a complex multi-stage evolution. In contrast, the high-K calc-alkaline dacites and rhyolites have less enriched initial Sr (0·7091-0·7097) and Nd (0·51213-0·51225) isotopic compositions. The presence of zircon xenocrysts with a Pb-evaporation age of 471 ± 33 Ma documents the importance of crustal anatexis in their genesis. Processes responsible for the partial melting of metasomatized lithospheric mantle and post-collisional magmatism in the Lhasa block could be a consequence of (1) convective removal of the lower lithosphere or (2) of slab breakoff.

Keywords: lithospheric mantle;Sr-Nd-Pb-isotopes;SW Tibet; ultrapotassic volcanism

Pages 1399-1424